1. Field of the Invention
The present invention relates to a mirror controller of an optical switch in an optical cross-connect (OXC) system for wavelength division multiplexing (WDM) communication or, in particular, to a mirror controller suitably used for an optical switch using a MEMS (micro electro-mechanical system) mirror.
2. Description of the Related Art
An optical cross-connect system using the MEMS mirror is disclosed in Japanese Unexamined Patent Publications. No. 2002-236264 (Patent Document 1) and No. 2001-174723 (Patent Document 2).
Patent Document 1 proposes a device and a method for controlling an optical switch, in which the angular displacement of the MEMS mirror used for the optical switch is automatically corrected thereby to reduce the optical loss. This method is divided into a method in which part of an optical signal is branched by reflection on a mirror the like and the positional information of the branch light is obtained thereby to detect the angular displacement of the mirror, and a method in which the angular displacement of the mirror is detected by detecting the power of the optical signal output from an output port.
Patent Document 2, on the other hand, proposes a device and a method for detecting the rotational drift of the MEMS mirror element used in an optical switch. In order to optically detect the rotational drift of the mirror, in addition to the optical path for optical communication, a test light is radiated on the mirror and the drift of the mirror is detected using an optical monitor (camera, etc.).
In the technique described in Patent Document 1, part of the optical signal is branched, and the positional information of the branch light is obtained thereby to detect the angular displacement of the mirror. The problem, however, is that although the angular displacement of the mirror can be detected by the branch light, the insertion loss as an optical switch is increased by branching the output light. The insertion loss is an important factor of the performance of the optical switch and, therefore, an optical switch control device with a low insertion loss is required.
Also, in the technique described in Patent Document 1, the angular displacement is detected by detecting the power of the optical signal output from the output port, and the control operation is impossible without the optical output. In the initial state, i.e. not yet optically connected, or in the case of some macroscopic variation, therefore, the optical connection cannot be controlled.
Further, in the technique of Patent Document 1, the optical signal input from an input collimator is reflected on an input mirror, and after being reflected on an output mirror, is output from the desired output collimator. The optical output level of the optical signal thus output is monitored by a photodiode or the like. In this way, the point or level is optimally controlled. In the case where the displacement of the initial angle is considerable, however, the problem is posed that the optical signal is not output from the output collimator and therefore the optical output cannot be monitored using the photodiode or the like.
Patent Document 2, on the other hand, discloses a technique for detecting the drift. The technique of Patent Document 2, however, has the problem that an optical path for the test signal other than that of the actual optical signal is required for the monitor operation, and any trouble of the optical signal due to the fault of the input fiber or the angular error between the input light and the test signal which may occur cannot be detected. In the case where the optical switch is used for WDM transmission, a very high reliability is required. The MEMS mirror, however, is not yet sufficiently reliable, and therefore the technique of Patent Document 2 using an optical path for the test signal, in addition to the light path of the actual optical signal, cannot solve the problems described above. Also, the technique of Patent Document 2 deals with the position of a MEMS mirror array but not the relation with the actual optical signal (the optical signal passing through a light path that can be switched by a mirror).